Choose add-or-replace when installing ProxUI's SSH key

[greenlogles]

Summary

Fixes multiple ProxUI instances clobbering each other's SSH access for cloud-init snippet writes.

Previously, installing ProxUI's snippet-write key ran grep -v 'proxui-snippet@' first, stripping every ProxUI key before adding its own — so a second instance wiped the first instance's access to the node.

Provisioning (Cloud-Init → Custom Snippet → Set up access) now offers a choice:

• Keep them, add this key (default) — removes only this instance's own key (matched by its key material, so re-runs stay idempotent) and leaves other instances' keys in place. Multiple ProxUI instances coexist.
• Replace all ProxUI keys with this one — removes every proxui-snippet@ key, for cleaning up keys from decommissioned instances.

The replace flag flows through /api/snippets/ssh/setup → _provision_ssh_key(..., replace=) → _install_key_command(..., replace=).

Notes

• Default is "add" because it fixes the multi-instance clobber by default and behaves identically to the old code on a clean node.
• Dedup for "add" is by the key's unique material, so re-provisioning the same instance never duplicates, and two instances that share a hostname still coexist.

Testing

• 167 unit tests pass; black clean.